Window Energy Ratings Explained: A to A++ and U-Values

Window quotes are full of letters and numbers that look interchangeable. They are not, and two quotes with the same letter can be very different products.

The rating scale

In the UK, window energy performance is rated on a scale from A++ (most efficient) down to E, under a scheme run by the British Fenestration Rating Council. Ratings appear on a label with a colour bar, similar in feel to a fridge rating.

Crucially, the rating covers the whole window: the frame material and profile, the glass, the spacer bars and the gas fill, all assessed together for heat loss, draughts and solar gain. That is why a cheap frame with good glass can rate worse than a better frame with the same glass.

What a U-value means

A U-value measures how quickly heat passes through a material, in watts per square metre per degree of temperature difference. Lower is better.

A U-value on its own does not tell you how efficient the window will be in use, because it ignores draughts and solar gain. That is exactly why the letter rating exists: it bundles those in. If a quote gives you only a U-value, ask for the rating as well.

The upgrades that earn a good rating

• Low-E coating: a microscopic metallic layer that reflects heat back into the room while letting light through. Standard on all new windows; there are better and worse grades.
• Argon or krypton fill: heavier than air, so it conducts less heat across the cavity. Usually included, sometimes an upgrade.
• Warm-edge spacers: the bar separating the panes. Metal spacers conduct heat and cause cold edges and condensation; warm-edge versions do not.
• Wider cavity: 16 mm to 20 mm is the efficient range in a double-glazed unit. More is not better.
• Triple glazing: a third pane and second cavity, giving a lower U-value. Around 20% more expensive than double, but the bill saving rarely justifies it alone.

A, A+ or A++?

For a typical UK home, A-rated double glazing is the sensible specification. A+ and A++ windows exist and are more efficient, but the difference in annual bill terms is a few pounds a year, while the premium can be substantial. If you have a very exposed site, or you are specifying triple glazing for noise, the higher ratings come as part of the package.

What to check on the quote

• The rating and the U-value, stated for the whole window rather than the sealed unit.
• The glass specification, for example 4 mm low-E / 20 mm argon / 4 mm, so you can compare like for like.
• The spacer type: warm edge or not.
• Whether trickle vents are included, and whether the rating depends on them being fitted.
• The frame profile and make, not just the material.
• The guarantee on the sealed unit, separately from the frame.

If two quotes differ by thousands of pounds for what looks like the same window, the specification is almost always where the difference lives. Ask both installers for these six answers in writing and the comparison becomes obvious.